The present invention relates to image stabilization for an imaging device, and more particularly to an image stabilizing optical device having a variable-angle optical lens for optically performing image stabilization.
In an imaging device such as a television camera, there is a problem of image shake due to shaky hand movements. Particularly in an imaging device having an imaging optical system having a zoom function, the characteristics of image shake are different according to the magnification of the optical system, so that it is difficult to effectively prevent the image shake over an adjustable range of a focal length.
A related art image stabilizing mechanism will now be described with reference to FIGS. 14A to 16B. The related art image stabilizing mechanism is generally classified into an electronic mechanism of cutting an image frame from an imaging area of a CCD 21 as shown in FIGS. 14A and 14B and an optical mechanism of adjusting an optical axis angle of incident light by combining a plano-convex lens 24 and a plano-concave lens 25 as shown in FIGS. 16A and 16B. In both the electronic mechanism and the optical mechanism, image shake is detected by an angular velocity sensor and is corrected for according to a detected value from the sensor.
The electronic mechanism will first be described with reference to FIGS. 14A and 14B. As shown in FIG. 14A, the CCD 21 used in the electronic mechanism has a large imaging area A0 with horizontal scanning lines larger in number than those defined by the video standard of a television. An actual video is obtained from a video signal by cutting from the area A0 an area A1 having the same number of horizontal scanning lines as that defined by the video standard. In correcting for image shake due to shaky hand movements, the area A1 is moved in the range of the area A0, for example, to an area A2 or an area A3, according to a shake detection signal, so as to eliminate image shake on the CCD 21, and the resultant area A2 or A3 is then cut from the area A0.
The correction power of the above electronic mechanism will now be described with reference to FIGS. 14A and 14B.
With regard to a vertical direction, letting f denote the focal length of an imaging lens 22, 2h0 denote the length of one side (the height) of the area A1, and 2(h+h0) denote the length of one side (the height) of the area A0, a correctable angle .theta. is given as follows: EQU tan(.theta.0+.theta.)=(h+h0)/f (1) EQU tan .theta.0=h0/f (2)
Since .theta.0+.theta. is small, EQU .theta.0+.theta.=(h+h0)/f (3) EQU .theta.0=h0/f (4)
Therefore, EQU .theta.=h/f (5)
Thus, a maximum correction angle can be obtained.
In the case that the CCD 21 has a size of 2/3 inch, the size of the area A0 becomes 8.8 mm.times.6.6 mm. In this case, a marginal area of correction has a horizontal length of 2.64 mm and a vertical length of 1.98 mm, provided that each length is about 30% of the length of the corresponding side of the area A0. Accordingly, a correction area as the half of the marginal area has a horizontal length of 1.32 mm and a vertical length of 0.99 mm. In the case that the focal length f of the imaging lens 22 is a shorter focal length of 8 mm, the correction angle .theta. for the vertical direction becomes 0.99/8.apprxeq.0.124 rad.apprxeq.7 degrees. Thus, a large correctable angle can be obtained. However, in the case that the focal length f is a longer focal length of 200 mm, the correction angle .theta. for the vertical direction becomes 0.99/200.apprxeq.0.005 rad.apprxeq.0.28 degree. Thus, the correctable angle becomes extremely small. Of course, the same can also apply to the horizontal direction.
Furthermore, according to this electronic image stabilization, the CCD 21 must ensure a large imaging area for correction. As a result, a chip size becomes large to cause an increase in cost. If a CCD conforming with the video standard is used, all the pixels of the CCD cannot be used to unavoidably result in degradation of image quality.
The optical mechanism will now be described with reference to FIGS. 15 to 16B. FIG. 15 shows an example of the optical mechanism. The optical mechanism shown in FIG. 15 has a prism 23 having a variable apex angle located in front of an imaging lens 22. The apex angle of the prism 23 is varied according to a shake detection signal to thereby adjust an optical axis of incident light.
The prism 23 having the variable apex angle is located in front of the imaging lens 22 in such a manner that one surface of the prism 23 is perpendicular to the optical axis. Letting .alpha. denote the apex angle of the prism 23, .delta. denote the deflection angle of outgoing light from the prism 23, and n denote the refractive index of the prism 23, the following equation is given. EQU sin .theta.=sin(.alpha.+.delta.)=n sin .alpha. (6)
Since .theta. is small, EQU .theta.=.alpha.+.delta.=n.alpha. (7)
Therefore, EQU .delta.=(n-1).alpha. (8)
Eq. (8) indicates that when the apex angle .alpha. is varied .+-.2 degrees in the case of n=1.5, for example, the deflection angle .delta. can be changed .+-.1 degree. The change of .+-.1 degree of the deflection angle .delta. is a sufficient value for correction of a shake angle actually occurring in the case that the focal length f of the imaging lens 22 is a longer focal length. However, in the case that the focal length f is a shorter focal length, e.g., f=8 mm, a correction amount becomes 8.times.tan 1.degree., which is an extremely small value of 0.14 mm on an image sensor in comparison with 0.99 mm in the electronic mechanism mentioned above.
In this manner, according to the above optical mechanism, the optical axis correction angle by the prism 23 located in front of the imaging lens 22 is constant irrespective of the focal length of the imaging lens 22. Accordingly, the optical axis correction angle is not reduced even in the case of a longer focal length, and effectively functions particularly in a zoom-in condition highly sensitive to shaky hand movements. However, in the case of a shorter focal length, a sufficient correction angle cannot be obtained.
Further, while the prism 23 contains a sealed liquid having a constant refractive index to vary the apex angle, it is difficult to increase the size due to viscosity of the liquid, and a problem on high-speed response performance is also present.
FIGS. 16A and 16B show another example of the optical mechanism. As shown in FIG. 16A, the optical mechanism has an optical system configured by combining a plano-convex lens 24 and a plano-concave lens 25 in such a manner that a convex surface of the lens 24 faces a concave surface of the lens 25 having the same curvature as that of the convex surface of the lens 24. As shown in FIG. 16B, the correction for image shake is performed by moving the plano-concave lens 25 in a direction depicted by an arrow Re to thereby equivalently vary the apex angle .alpha. of the prism 23 shown in FIG. 15, thus changing an angle of incidence upon an imaging lens 22. This technique has already been disclosed in Japanese Patent Laid-open Nos. Sho 59-26930 and Hei 6-281889. However, also in this example, there are problems such that large aberrations occur and a drive mechanism is complicated.